[llvm] [WIP][RISCV] Support 3-argument associative add for transformAddShlImm (PR #86883)

[Philip Reames via llvm-commits]

preames wrote:

So status here is clear, I'm waiting until the migration to using RISCVISD::SHL_ADD is complete before pursuing this.  I'm worried about combine loops with the current structure.  I'll ping once this is ready to move forward.

https://github.com/llvm/llvm-project/pull/86883